Video report by ITV News Political Correspondent Romilly Weeks
Organisers claim 670,000 people have attended a march in central London calling for a fresh referendum on the outcome of the Brexit negotiations.
The People's Vote march set off from Park Lane before heading to Parliament Square for a rally.
Protesters came from all over the UK to take part in the march, with coaches leaving places as far away as Devon at 6am to bring 400 people to the capital.
MPs from all the main political parties took part in the march, along with London Mayor Sadiq Khan, and celebrities such as Delia Smith, Dragons’ Den businesswoman Deborah Meaden and The Lord of the Rings star Andy Serkis.
Addressing the crowds in Parliament Square, Mr Khan accused the Government of leading the UK "towards either a bad Brexit deal, or even worse, no deal at all.
"These options are a million miles away from what was promised.
"The Government doesn't have a mandate to gamble with our future...
"The Government is risking the livelihoods of our children without the people having a final say. Are we going to stay quiet?
"The Government is choosing party politics over the national interest. Are we going to let them get away with it?
"The lies, the mistruths, the deceptions are now being exposed and the will of the British people is changing...
"No one who voted to leave the EU voted to make themselves poorer.
"No one voted to make it more difficult for their children and grandchildren.
"No one voted to damage the NHS.
"No one voted for what this Government is leaning towards.
"Those that complain and say that a public vote is undemocratic, is unpatriotic; realise in fact that the exact opposite is the truth.
"What can be more democratic, what can be more British than trusting the judgement of the British people?"
Children who took part in the People's Vote march tell ITV News why they did
Conservative MP Anna Soubry also addressed the rally, telling crowds in Parliament Square that "it is clear we are the many".
She continued: "We are winning the argument, most importantly against those who voted leave...
"We will take responsibly and sort out this mess."
While in his speech to the rally, Sir Vince Cable, leader of the Liberal Democrats, said it was "a tragedy this country is being divided by generation".
He said it was the "majority" of his generation that voted to leave, "taking the freedom" away from young voters.
Receiving cheers from the crowds, he said: "There is no deal better than the one we have now: it is better for Britain and better for Europe."
Following his speech, Sir Vince added: "I think people have woken up to the potential disaster.
"Even if they negotiate a deal, it's going to be a bad deal, where we're going to spend years under European Union rules but have no say in them and beyond that there's a cliff edge.
"We've realised there isn't a good deal coming out of this and a lot of people are frightened, people are worried."
In a video message of support, Nicola Sturgeon, the First Minister of Scotland, said: "Let me say this loudly and clearly, if the issue comes before the House of Commons, SNP MPs will support a People's Vote which includes the option to remain in the EU."
She added: "The Tory government's handling of these negotiations has been chaotic, incompetent and shambolic.
"Having spent two years telling us that no deal was better than a bad deal, the Prime Minister is now preparing to pile pressure on MPs to vote for a bad or blindfold deal on the grounds that 'no deal' would be catastrophic.
"She is trying to scare the UK into the frying pan out of fear of the fire. It is a scandal and it should not be accepted."
Delia Smith said people were not fully informed when they voted but now understood "the dire consequences".
"The only way we can avoid this total madness and win back our future has to be a People's Vote."
The march was led by a group of young voters calling for a second referendum.
Emily Longman, 20, said she was four months too young to vote in the referendum.
Miss Longman, who was marching alongside Emma Stevens, also 20, said: "We're both Spanish students due to study abroad next year, but no one knows what will happen with Erasmus funding."
Joe Trickey, from Croydon, was celebrating his 83rd birthday at the march.
He said: "I believe very strongly in the EU as a place of peace and strength.
"Going out puts us in isolation and leaving isn't about trade deals, it's about our values."
Jason Gillot originally voted to leave the EU, but said he changed his mind five days after the referendum.
The 43-year-old, from London said: "I'm politically agnostic but I was just sick of the lies that have come out of both sides."
He said he initially voted to leave due to "economic evidence partly supplied by the Tax Payers' Alliance" which he says "made sense at the time."
"Now we have actual facts and realities of what's going to be happening," he added.
Mr Gillot has been marching with a sign that says: "When the facts change, I change my mind! What do you do sir?"
A pro-Brexit "Leave Means Leave" rally is also taking place in Harrogate, with former UKIP leader Nigel Farage, Labour MP Kate Hoey and Tory MP Owen Paterson attending.
Theresa May has repeatedly ruled out holding a second referendum.
Meanwhile in Northern Ireland, a separate pro-Remain rally was held, with organisers telling the crowd in Belfast that the EU is a force for peace.
Hundreds of people waving EU flags and carrying anti-DUP placards converged outside the city hall on Saturday afternoon.
Addressing the crowds, Cross-community Alliance Party leader Naomi Long said: "We have the EU to thank for the longest period of peace and stability on the continent of Europe in history...
"Nowhere did it do that more so than right here."
She said the Brexit debate was not about protecting the UK's union or creating a united Ireland, as it has been characterised by some on opposite sides of the issue.
Mrs Long said: "This is about the people of this place coming together and saying, just like the EU, we value cooperation, we value immigration, we value working together in the best interests of everyone in this society.
"That is why we value the EU, it is not just a model of cooperation, it underpins the very cooperation that we need here."
Northern Ireland voted to remain in the EU by 56%.
The crowd held a range of placards, including one proclaiming: "We won't be DUP'ed" in reference to the pro-Brexit Northern Irish party's partnership with Theresa May's Conservatives at Westminster.
Other slogans demanded a People's Vote, a second referendum, which another podium speaker, SDLP South Belfast Assembly member Claire Hanna, said Brexiteers should have nothing to fear from if they are so wedded to democracy.
Meanwhile, Sinn Fein vice-president Michelle O'Neill said Brexiteers had exhibited "reckless disregard" for the people of Northern Ireland.
"They care nothing for jobs or for rights and are prepared to drive our economy over the cliff."